A known example of a recording system that uses a recording head including a plurality of recording elements for recording dots may be an inkjet recording system that records dots on a recording medium by ejecting ink from recording elements (nozzles). Inkjet recording apparatuses of such a system may be classified into a full-line type and a serial type depending on the difference between configurations of the types. With any one of the full-line type and serial type, an ejection quantity and an ejection direction may vary among the plurality of recording elements of the recording head. Then, the variation may cause density unevenness and lines to appear in an image.
A known technique that decreases the density unevenness and lines may be a multi-pass recording system. The multi-pass recording system divides image data that is recorded in a single region of a recording medium, into image data that is recorded by a plurality of recording scans. The divided image data is successively recorded by the plurality of recording scans with a conveyance operation. Accordingly, even if ejection characteristics of individual recording elements vary, dots that are recorded by a single recording element are not continuously arranged in a scanning direction. Influences of the individual recording elements can be spread in a wide range. Consequently, a uniform and smooth image can be obtained.
The multi-pass recording system may be applied to a recording apparatus of the serial type or a full-multi type including a plurality of recording heads (a plurality of recording element groups) that eject ink of a single type. In particular, image data is divided into image data that is recorded by the plurality of recording element groups for ejecting the ink of the single type, and the divided image data is recorded by the plurality of recording element groups during at least a single relative movement. Consequently, even if the ejection characteristics of the individual recording elements vary, the influences of variation can be reduced. Further, the above-mentioned two recording methods may be combined, to record an image by the plurality of recording scans with the plurality of recording element groups for ejecting the ink of the single type.
Conventionally, when image data is divided, a mask is used. The mask has data that permits recording of a dot (1: data that does not mask image data) and data that does not permit recording of a dot (0: data that masks image data), the former data and the latter data being previously arranged in the mask. To be more specific, logical multiplication is performed between the mask and binary image data that is recorded in a single region of a recording medium. Thus, the binary image data is divided into binary image data that is recorded by the recording scans or recording heads.
In such a mask, the arrangement of the recording permissive data (1) is determined such that the plurality of recording scans (or the plurality of recording heads) are complements of each other. That is, a dot is recorded on a pixel, on which binarized image data is recorded (1), by one of the recording scans or one of the recording heads. Hence, image information before the division can be saved even after the division.
However, the multi-pass recording may cause another problem, such as density variation or density unevenness due to a deviation between recording positions (registration) on a recording scan basis or a recording head (recording element group) basis. The deviation between the recording positions on a recording scan basis or a recording element group basis represents a deviation as follows. The deviation is a deviation between dot groups (planes), i.e., a deviation between a dot group (plane) recorded by a first recording scan (or one recording element group) and a dot group (plane) recorded by a second recording scan (or another recording element group). The deviation between the planes results from, for example, a variation in distance (gap) between a recording medium and a surface with ejection ports, or a variation in conveyance distance of the recording medium. If the deviation between the planes occurs, a dot coverage may vary, and this may result in the density variation and density unevenness. Hereinafter, like the case described above, a dot group or a pixel group that is recorded by a single recording scan with one means (for example, a single recording element group that ejects a single type of ink) is called “plane.”
In light of the situations, since a high-quality image is being desired, a method for processing image data for multi-pass recording is requested, the method which can deal with a deviation between recording positions of planes resulting from variations in various recording conditions. Hereinafter, no matter which recording condition causes the deviation between the recording positions of the planes, a resistance to the density variation and density unevenness resulting from the deviation is called “robustness” in this specification.
Patent Literatures 1 and 2 each disclose a method for processing image data to increase robustness. Those literatures focus on that a variation in image density resulting from variations in various recording conditions occurs because binary image data, which is distributed to correspond to different recording scans or different recording element groups, is fully complements of each other. If image data corresponding to different recording scans or different recording element groups is generated such that the complementary relationship is degraded, multi-pass recording with good “robustness” can be provided. Regarding those literatures, to prevent significant density variation from occurring even if a plurality of planes are shifted from each other, multivalued image data before binarization is divided into data corresponding to the different recording scans or recording element groups, and the divided multivalued image data is individually binarized.
FIG. 10 is a block diagram showing the method for processing image data described in each of Patent Literature 1 and 2. The method distributes multivalued image data into two recording scans. Multivalued image data (RGB) input from a host computer is converted by palette conversion 12 into multivalued density data (CMYK) corresponding to ink colors provided in a recording apparatus. Then, gradation correction (13) is performed for the multivalued density data (CMYK). The following processing is performed individually for black (K), cyan (C), magenta (M), and yellow (Y).
Multivalued density data of each color is distributed by image data distribution 14 into first-scan multivalued data 15-1 and second-scan multivalued data 15-2. In particular, if a value of multivalued image data of black is “200,” “100” which is a half the “200” is distributed for a first scan, and “100” which is the other half is distributed for a second scan. Then, the first-scan multivalued data 15-1 is quantized by first quantization 16-1 in accordance with a predetermined diffusion matrix, converted into first-scan binary data 17-1, and stored in a first-scan band memory. Meanwhile, the second-scan multivalued data 15-2 is quantized by second quantization 16-2 in accordance with a predetermined diffusion matrix that is different from the matrix of the first quantization, converted into second-scan binary data 17-2, and stored in a second-scan band memory. During the first recording scan and the second recording scan, ink is ejected in accordance with the binary data stored in the band memories. The case in which the single image data is distributed into the two recording scans has been described with reference to FIG. 10. Also, Patent Literatures 1 and 2 disclose a case in which single image data is distributed into two recording heads (two recording element groups).
FIG. 6A illustrates an arrangement state of dots (black dots) 1401 recorded by a first recording scan and dots (white dots) 1402 recorded by a second recording scan when image data is divided by using mask patterns that are complements of each other. Herein, density data of “255” is input for all pixels, and every pixel has a single dot recorded thereon by either the first recording scan or the second recording scan. That is, the dots recorded by the first recording scan and the dots recorded by the second recording scan are arranged not to overlap each other.
FIG. 6B illustrates an arrangement state of dots when image data is distributed by the method disclosed in each of Patent Literatures 1 and 2. In the drawing, black dots are dots 1501 recorded by the first recording scan, white dots are dots 1502 recorded by the second recording scan, and gray dots are dots 1503 redundantly recorded by the first recording scan and the second recording scan. In FIG. 6B, the dots recorded by the first recording scan and the dots recorded by the second recording scan are not complements of each other. Therefore, as compared with FIG. 6A with the fully complementary relationship, there are the part (the gray dots) 1503 in which two dots overlap each other, and a white region in which no dot is recorded appear.
Here, a case is considered in which a first plane that is a set of dots recorded by the first recording scan is deviated in a main-scanning direction or a sub-scanning direction by a single pixel from a second plane that is a set of dots recorded by the second recording scan. At this time, if the first plane and the second plane are complements of each other as shown in FIG. 6A, the dots recorded in the first plane completely overlap the dots recorded in the second plane, the white region is exposed, and the image density is markedly decreased. Although the deviation is smaller than the single pixel, if a distance between adjacent dots or an overlap part varies, the variation may significantly affect the dot coverage to the white region and even the image density. In other words, if the deviation between the planes varies because of the variation in distance (gap) between the recording medium and the surface with ejection ports or the variation in conveyance distance of the recording medium, a uniform image density may vary, and the variation is recognized as the density unevenness.
In contrast, in the case in FIG. 6B, although the first plane is deviated from the second plane by a single pixel, the dot coverage to the recording medium does not significantly vary. A part in which the dots recorded by the first recording scan overlap the dots recorded by the second recording scan may newly appear; however, a part, in which the redundantly recorded two dots are separated from one another, may also appear. Thus, regarding a region with a certain size, the dot coverage to the recording medium does not markedly vary, and the image density hardly varies. That is, with the method in each of Patent Literatures 1 and 2, even if the distance (gap) between the recording medium and the surface with ejection ports varies, or if the conveyance distance of the recording medium varies, the variation in image density and the occurrence of the density unevenness can be suppressed, and an image with good robustness can be output.